Actually, now that I think about it....
That looks like an early Aeron that I had on a AirArms S400.
When they first started making those stocks, I think that they were for the AA 400 and 500 series maybe?
Mine came from a guy called Peter Plessinger at the old TopGun.
The same stock was also offered by AOA for the FX Royale series of rifles until a few years ago.